84 McQuay OM 920-1
Operator’s Guide
Bypass Dampers (Not Applicable for 100% OA Units)
• The Bypass Dampers are driven closed (Bypass Damper Closed output is energized)
whenever the OA Damper position is less than or equal to the Minimum OA Position
• The Bypass dampers are driven open (Bypass Damper Open output is energized) whenever
the OA Damper Position exceeds the Minimum OA Damper Position by more that 3%
Outside Air Damper Control
Minimum Outside Air Damper Control
Control of the dampers in the Economizer state is described in the Economizer Control
section. The outdoor air dampers are driven open in the Mechanical Cooling State if
economizer operation is enabled and to the Minimum OA Position if economizer operation is
disabled. For all other operating conditions, the outdoor air dampers are set to the Minimum
OA Position. The Minimum OA Position is set to zero or closed position when the Supply Fan
is off, the unit is in the Recirculation state, Occupancy is set to Unocc, or the fan has been on
for less than the Zero OA Time.
As a result, the OA dampers are driven closed in night setback, night setup, morning warm-up,
and morning cool down situations unless economizer operation is required. In all other
conditions the Minimum OA Position is equal to or below a Ventilation Limit and equal to or
above a Demand Control Ventilation limit. For CAV units, the Ventilation Limit equals the
keypad editable Vent Limit and the Demand Control Ventilation limit equals the keypad
editable DCV Limit. For VAV units, the Ventilation Limit varies with VFD speed between the
editable Vent Limit at 100% Supply Fan speed and the editable LoFlo Vent Limit at the Min
Clg Spd. For VAV units, the Demand Control Ventilation Limit equals the Ventilation Limit
times the ratio of the editable DCV Limit to the editable Vent Limit.
The editable parameters are to be determined when the airflow for the unit is balanced and are
located in the Standard Menu/Min OA Damper menus.
Note – The MinClgSpd is prevented from being set equal to the Design Cooling Speed. The DCV
Limit is prevented from being set greater than the Vent Limit. If the VentLimit or the
LoFloVent Limit is set to 0, the Ventilation Limit is overridden to 0.
Cold Start Operation
A special "cold start" sequence will slow the opening of the dampers when it is cold outdoors
and the unit is equipped with either Hot water/Steam or F&BP heating. This is to try to
prevent nuisance freezestat trips associated with dampers opening up rapidly to minimum
position before the heat has a chance to ramp up. This "cold start" sequence is initiated if the
following conditions are all true:
• OAT is below the current LoDATLimit
• The unit is equipped with and Airside Economizer
• The current Unit State is greater than Recirc
• The current Minimum Outdoor Damper Position set point is greater than 0%
• The unit is equipped with Hot water/Steam or F&BP heating
When the sequence is active the dampers will move more slowly the colder it is outdoors. The
minimum and maximum ramp rates are adjustable via the keypad by navigating to the
Extended Menus/Economizer setup menu. The effective ramp rate will vary from the
minimum rate at OAT equal to -30°F to the maximum at OAT equal to 100°F. Once the
damper position reaches a point 1% below the actual effective minimum position normal
operation will begin. If the unit enters the Economizer operating state before the damper
regulation begins, the regulation will begin from the current economizer position.
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